A switch device as identified by numeral 101 in FIGS. 6(a) and 6(b) has been employed in the past for starting a motor and for protecting the motor against over-current conditions. Switch device 101 comprises a protection device 108 and a resistance element 109 for starting, with one end of the protection device 108 being connected to the power line L1 and the other end connected to a common terminal C of a motor 110. One end of resistance element 109 is connected to a start winding terminal S of motor 110 and the other end is connected to ground line L2. A main winding and a start winding (not shown in the drawings) are provided inside motor 110, the ends of the main winding being connected to common terminal C and the main winding terminal M respectively and the ends of the start winding being connected to common terminal C and the start winding terminal S, respectively.
A switch circuit 111 and a resistive heater 113 are provided inside the protection device 108 and, prior to starting motor 110, the switch circuit 111 is in an electrically conductive state. Accordingly, the common terminal C is connected, in that state, to power line L1 through switch circuit 111. The main winding terminal M is directly connected to ground line L2 and, at the time when motor 110 is to be started, voltage is impressed between power line L1 and ground line L2 with the result that voltage is impressed between the common terminal C and the main winding terminal M and the start winding terminal S. Resistance element 109 is a PTC (positive temperature coefficient of resistivity) element having a low resistance at normal or room temperature and which, above an anomaly temperature, increases in resistance as temperature increases. Since it has low resistance at start-up, when voltage is impressed between the common terminal C and main winding terminal M and the start winding terminal S, a large electric current flows to the start winding, thereby assisting the starting phase. When a large electric current flows to the start winding, motor 110 starts rotating and, as its rate of rotation increases, the electric current that flows to the start winding becomes unnecessary. During the starting phase, resistance element 109 is heated by the electric current that flows to the start winding and the resistance value increases. As a result of this, the electric current that flows to the start winding is gradually reduced to a low level but it does not become zero. Switch 101 has a switch arm 112, a contact 115 and a resistive heater 113 provided in the first switch circuit 111 for the electric current that flows through the main winding and the start winding. A bimetal element 114 is provided in close proximity to resistive heater 113 inside protection device 108 so that the electric current which flows through resistive heater 113 generate heat which will heat bimetal element 114 raising the temperature of the bimetal. In as much as the resistive heater 113 has a low resistance value however, the amount of heat generated by the electric current that flows when motor 110 is started or driven is small and the increase in temperature of bimetal element 114 is small as well.
In the event that an excessive electric current flows to motor 110 due to some fault condition, a large amount of heat is generated by resistive heater 113, with a result that bimetal element 114 is heated to a high temperature. Regarding the shape of bimetal element 114, the side which faces arm 112 is concave facing arm 112 at normal temperatures but, if it is heated to an actuation temperature, e.g., 145.degree. C., by the heat that is supplied from resistance heater 113, it snaps to the opposite, convex configuration facing arm 112 as is shown in FIG. 6(b), thereby raising arm 112 and bringing contact 115 into an open state. When the contact 115 assumes an open state, current flow to the motor 110 is terminated.
According to the switch device of the prior art, starting of motor 110 is carried out by PTC element 109 and protection against over-current is carried out by protection device 108 as described above. Motor 110 is protected relative to an electric current of a selected quantity which can be varied by changing the base resistance value of the PTC element, changing the start-up characteristics, changing the resistance value of resistive heater 113, or changing the shape or the actuation temperature of bimetal element 114. Since the electric current that flows to the start winding is controlled by the resistance change of resistance element 109 in the above switch 101, however, it becomes necessary to increase the volume of the element in order to make the resistance value of resistance element 109 sufficiently small at the time of the start-up, with a result that there are cases where the starting characteristics of motor 110 deteriorate as the starting electric current is restricted by resistance element 109. During operation of motor 110, moreover, resistance element 109 becomes highly resistant. As some electric current continues flowing to the start winding through resistance element 109 even during operation of motor 110, however, there arises a problem in that the operating efficiency of motor 110 is decreased.